Radio receiving system



Se t. 28, 1937. K. A. CHITTICK RADIO RECEIVING SYSTEM Filed Sept. so, 1955 2 Sheets-Sheet '1 2w on F3 QLRDW KNE- 01 Ken-n h fLChittich o-QJAbMo-u ATTORNEY Sept. 28, 1937.

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"K. A. CHlTTlC-K RADIO RECEIVING SYSTEM Filed Sept. 30, 1933 2 Sheets-Sheet 2 x 1 53 I 4 a 3 45 i 1 1 I Ken ethflchitticfi,

.BY 5 Q Q V ATTORNE IN VENTOR Patented Sept. 28, 1937 UNITED srarss A'l ENT CFFICE RADIO RECEIVING SYSTEM Kenneth A. Chittick, Haddonfield, N. J., assignor to Radio Corporation of America, a corporation of Delaware My invention relates to radio receiving sys tems, and more particularly to radio receiving systems of the type provided with automatic control means.

It has been found that radio receiving systems,

even when controlled by automatic volume control means may not always function as normally intended when subjected to strong signals, as

provided by a powerful local transmitter for example, for the reason that the first amplifier stages and the amplifier devices therein may not be effectively prevented from becoming overloaded.

Overloading of the first stages and amplifier devices may often occur for the reason that the ordinarily necessary selectivity of the system preceding the automatic volume control circuit may be so high that when the receiver is tuned slightly to one side of the carrier wave of a strong local signal, insuificient signal energy therefrom may be supplied to the volume control circuit to provide proper control potentials. Under such conditions, however, the tuning of the first amplifier stages is ordinarily sufiiciently broad to permit enough of the signal from the local station to reach the first amplifier devices or tubes to overload them.

Various expedients have been resorted to in order to prevent overloading of the first or input tubes of a radio receiving system provided with automatic volume control means. For example, the ratio of gain to selectivity in the initial amplifier stages may be kept below a certain value determined by the tendency of the particular amplifier device therein to overload on strong signals.

In other receiving systems, the automatic volume control circuit may be connected to an 40 amplifier stage nearer the antenna circuit than it would be except for the problem of preventing the first tubes from overloading. Connection with an earlier amplifier stage is made because the selectivity is less there than at a later stage 5 and, consequently, enough energy to 'make the volume control circuit function may be supplied thereto even when the receiver is tuned slightly out of a strong carrier wave. It will be apparent that there are disadvantages in employing the 50 above mentioned and other similar expedients for the same purpose.

It is, accordingly, an object of my invention to provide an improved radio receiving system in which the initial amplifier devices are effectively 55 prevented from becoming overloaded without. re:-

A still further object of my invention is to provide a. radio receiving system in which successive amplifier stages are provided with individual'vol- Lime and selectivity control circuits.

In practicing my invention, I provide means for broadening the tuning of the tube circuits 20 of the radio receiver in response to an increase in signal strength; and more specifically I provide means for broadening the tuning of the circuits remote from the antenna circuit more, with a given increase in signal strength, than the tuning 25 of the preceding circuits. In an ideal embodiment of my invention the circuit at the antenna has the greatest selectivity, while the succeeding circuits have progressively less selectivity.

In applying my invention to certain types of 30 radio receiver circuits and in particular to a superheterodyne circuit, it is generally impractical to make the first tuned circuits of the re ceiver as selective as the succeeding tuned circuits. This is for the reason that the tuned radio frequency circuits in the antenna end of that type of receiver are inherently less selective than the intermediate frequency stages'which are tuned to the lower or intermediate frequency.

By broadening the tuning of the intermediate 40 frequency tuned circuits during the reception of strong signals, and, particularly, by broadening the tuning of the intermediate frequency circuits the most, sufficient energy from received signals may reach the automatic volume control means, 5 usually connected with the output circuit of an intermediate frequency amplifier stage to lower the gain of the first tubes even though the receiver is tuned slightly off the carrier of a strong signal.

The automatic control of the selectivity and gain of an amplifier or detector stage is obtained by utilizing electric discharge devices or electronic tubes of the suppressor grid type and by applying a control voltage to the suppressor grid.

It will be understood that in this type of tube the suppressor grid is a grid positioned between the screen grid and the anode. In conventional circuits, the suppressor grid is connected directly to the cathode.

The control voltage for the suppressor grid is obtained by utilizing suppresor grid tubes which include a diode rectifier. A portion of the energy from the plate circuit of one tube is impressed across the diode rectifier of the succeeding tube, whereby a voltage drop is obtained in the rectifier circuit which may be impressed upon the suppressor grid of the one tube. t will be apparent that the energy supplied to succeeding rectifiers becomes rogressively greater, whereby the control voltage applied to the suppressor grids increases progressively from the antenna circuit to the last amplifier or detector stage.

Other features and advantages of my invention will appear from the following description, taken in connection with the accompanying drawings in which,

Figure 1 is a circuit diagram of a radio receiver constructed in accordance with one embodiment of my invention; and

Figs. 2 and 3 are circuit diagrams showing modified forms of my invention.

Referring to Fig. 1, there is shown the circuit of a superheterodyne receiver comprising a radio frequency amplifier tube I, a first detector tube 3, an intermediate frequency amplifier tube 5, and a second detector tube I.

The radio frequency amplifier tube I is of the suppress-or grid type and includes a cathode 9, a control grid II, a screen grid I3, a suppressor grid I5, and an anode II.

The input circuit of the amplifier tube I consists of a radio frequency transformer I9 having a primary 2! and a secondary 23, the primary 2! being connected to an antenna 25 and ground. The secondary 23 is shunted by a variable tuning condenser 27, which is one unit of a gang condenser, and connected to the control grid II at its upper end. A condenser 29 provides a radio frequency connection between the cathode 9 and the lower end of the secondary winding 23 which is connected to ground.

A suitable positive voltage is supplied to the anode I"! from the voltage divider SI of the power supply 33, the anode I! being connected to the upper end of the voltage divider 3I through a tuned coupling circuit 35 which includes an inductance coil 3'! shunted by a variable tuning condenser 39 which is another unit of the above mentioned gang condenser. All units of this gang condenser are tunable simultaneously from a single control.

The control grid I I is maintained at a negative potential by means of a biasing resistor M in the cathode lead.

The first detector tube 3 is also of the suppressor grid type, but in addition to the elements included in the radio frequency tube I, it includes a rectifier element 43 positioned adjacent to the lower end of the cathode 45. Thus, the rectifier element 43 and the lower end of the cathode 45 form a diode rectifier in the vacuum tube 3. A satisfactory electronic tube of this type is the RCA 237.

In addition to the cathode 45 and rectifier element 43 above mentioned, the first detector tube 3 includes a control grid M, a screen grid 49, a suppressor grid 5!, and an anode 53.

The upper end of the tuned coupling circuit 35 is coupled to the control grid 4'! through a coupling condenser 55. The lower end of the coupling circuit 35 is coupled to the cathode 45 through a circuit which may be traced from the coupling circuit 35 through the conductors 51 and 59, through the bypass condenser 6| to ground, and from ground through a bypass condenser 63 to the cathode 45.

The superheterodyne oscillator (not shown) is coupled to the control grid 47 of the first detector 3 through a transformer 65 having a secondary winding 57! included in the detector grid circuit. The radio frequency path of this grid circuit may be traced from the control grid 41 through a resistor 69, the secondary winding 67, a bypass condenser II to ground, and from ground through the bypass condenser 63 to cathode 45.

The grid-biasing direct current path of the detector grid circuit may be traced from the control grid i'l through the resistor 69, the secondary winding er, and a resistor 13 to ground, and from ground through a resistor 15 in the cathode lead to the cathode 45. The resistor I3 and condenser l'i function as a filter to filter out any hum introduced into the grid circuit by the grid biasing voltage.

The oscillator frequency is varied for tuning purposes by means of a. condenser (not shown) which is a unit of the above-mentioned gang condenser. It will be understood that any type of oscillator or method of coupling the oscillator to the first detector tube may be employed in practicing my invention.

In accordance with one embodiment of my invention, energy is taken from the plate or coupling circuit 35 of the vacuum tube l by means of an inductance coil ll coupled to the inductance coil SI of the coupling circuit 35 and having its upper terminal connected to the rectifier element 63 of the first detector tube 3. The lower end of the coil TI is connected to ground through a resistor l9.

When radio frequency energy above a certain value appears in the coupling circuit 35, a portion of this energy is transferred to the coupling coil TI and rectified by means of the diode rectifier in the vacuum tube The current path in the rectifier circuit is such that the upper end of the resistor I9 is negative with respect to ground. Obviously, no rectified energy appears in this rectifier circuit until the peak signal voltage therein becomes greater than the voltage drop in the biasing resistor 15. This provides the desired delay in the action of the automatic volume control.

The negative end of the resistor 19 is connected to the suppressor grid I5 of the radio frequency tub-e I through a filter resistor 8|. Filter condensers 83 and 535 are connected between the ends of resistor iii and ground for filtering out any audio frequency component or radio frequency component that appears in the rectifier circuit. The values of the filter elements are such that the control voltage applied to the suppressor grid I5 will change in accordance with slow variations in the strength of the incoming carrier wave. In addition to functioning as a filter element, condenser 83 acts as a-high frequency bypass condenser in shunt to resistor I9.

It will be seen that in the above-described circuit, as the strength of the incoming signal increases, the upper end of the resistor '19 becomes more negative and the negative potential applied to the suppressor grid i5 is increased.

An increase in the negative potential of the suppressor grid I5 causes a decrease in the gain of the amplifier I and also lowers its plate impedance. As indicated in Radiotron I-Iand Book H. B.-3, page 92, 0-4478, a pentode device such as a type RCA 58, R. F. pentode, when operated with an increasing negative bias on the sup pressor grid, has a decreasing plate impedance, the normal plate impedance being substantially .8 megohm with 0 volts on the suppressor grid. With volts negative bias on the suppressor grid, the plate impedance is approximately .4 megohm and this decreases to a minimum of about 50,000 ohms with between 30 and 40 volts negative bias after which the plate impedance increases withan increase in negative bias on the suppressor grid.

The decrease in plate impedance obviously broadens the tuning of the coupling circuit .35, since the plate impedance of the tube I is connected in parallel with the coupling circuit 35. It may be noted that, when the suppressor grid is made negative, the plate current is decreased slightly whereby the negative bias on the control grid is decreased. While this will tend to increase the gain of the tube, the effect will be negligible compared with the effect of the suppressor grid and will be in such a direction as to increase the broadening effect on the tuning.

The anode 53 of the first detector tube 3 is connected to the positive terminal of the voltage divider 3I through a coupling circuit 81. The coupling circuit 8! is tuned to the intermediate frequency signal which is produced in the usual way by beating, the incoming signal with the energy supplied by the superheterodyne oscillator. The coupling circuit In comprises an inductance coil 09 which is shunted by a tuning condenser 9|. It will be understood that the tuning of the circuit 81 is adjusted to some fixed frequency and that, in general, the intermediate frequency is lower than the frequency of the incoming signal.

' The intermediate frequency amplifier tube 5 is of the same type as the first detector tube 3. It includes a cathode 93, a control grid 95, a screen grid 91, a suppressor grid 99, an anode ml, and a rectifier element I03.

The control grid 95 of the amplifier tube 5 is coupled to the upper end of the coupling circuit 8'! by means of a coupling condenser I05. A radio frequency connection between the cathode 93 and the lower end of the coupling circuit 8? is provided by a circuit which may be traced from the cathode 93 through a bypass condenser I0! to ground, and from ground through the bypass condenser BI to the lower end'of the coupling circuit 81.

- The control grid 95 of tube 5 is provided with the proper negative bias by means of a biasing resistor I09 connected between the cathode 93 and ground, the control grid 95 being connected to a point on the biasing resistor I09 through a grid resistor III.

'It will be noted that, in this particular receiver, the bias applied to the grid is less than the total voltage drop along resistor I09. In other words, it is less than the delay voltage providedin the control circuit. The amount of delay voltage provided in the control circuits depends upon the conditions under whichthe receiver is to be operated.

In some cases, it may be desirable to omit the delay voltage entirely so that the broadening effect will begin immediately upon the reception of a signal. In omitting the delay voltage, the cathode is grounded and the grid is connected to a negative point on the power supply potentiometer in accordance with usual practice.

As in the preceding amplifier stage, an inductance coil II 3 is coupled to the inductance coil 89 of the coupling circuit 81 for the purpose of impressing radio frequency energy across the diode rectifier elements I03 and 93 of the amplifier tube 5. The diode rectifier circuit may be traced from the upper end of the coupling coil H3 through the rectifier element I03, and through the space current path of the rectifier 'tothe cathode 93, through the biasing resistor I09 to ground, and from ground through a resistor I I5 to the lower end of the coupling coil I I3. The suppressor grid 5| of the first detector tube 3 is connected through a filter resistor 1 to the upper end of the resistor II5, whereby the suppressor grid 5I is supplied with a negative potential having a value which depends upon the amount of energy transferred from the coupling circuit 8'7 to the rectifier coupling coil II3. Condensers H9 and IZI, connected between the ends of resistor III and ground, perform the same functions as the condensers 83 and 85 in the preceding stages.

More energy is transferred from the output circuit 81 of the first detector to its associated rectifier circuit than is transferred from the preceding output circuit 35 to its associated rectifier circuit since the signal has been amplified in the first detector. Consequently, a control voltage of greater magnitude is applied to the suppressor grid 5I of the first detector 3 than is applied to the suppressor grid I5 of the amplifier tube I.

Since an incoming signal makes the suppressor grid 5| of the first detector 3 more negative than the suppressor grid I5 of the preceding amplifier tube I, it is evident that the tuning of the detector output circuit 81 is broadened more as the signal strength increases than is the preceding tuned circuit 35. Also, the gain of the first detector tube 3 will be decreased more than is the gain of the radio frequency amplifier tube I.

The anode I III of the intermediate frequency amplifier tube 5 is connected to the positive terminal of the voltage divider 3| through another tuned intermediate frequency coupling circuit I23 which comprises an inductance coil I25 shunted by a tuning condenser H1.

The second detector tube I is of the same type as the preceding tubes 3 and 5 and includes a cathode I29, a control grid I3I, a screen grid I33, a suppressor grid I35, an anode I31, and a rectifier element I39.

The control grid I3! is coupled to the upper end of the coupling circuit I23 by means of a coupling condenser MI. The radio frequency connection between the cathode I29 and the lower end of the coupling circuit I23 may be traced from the cathode I29 through a bypass condenser Hi3 through the bypass condenser El and the conductors 59 and M5 to the lower end of the coupling circuit I23.

As in the preceding stage, the control grid I3I is supplied with a suitable negative bias by means of a biasing resistor I I'I connected between the cathode I29 and ground, the control grid being connected to a point on the biasing resistor I4 through a grid resistor I49.

As in the two preceding stages, the rectifier element I39 is connected to the upper end of a coil I5! inductively coupled to the inductance coil I25 of the coupling circuit I23. The lower to ground, and from ground end of the coil H5! is connected through a resistor I53 to ground, and from ground through the biasing resistor Bill to the cathode M9. The negative potential which will appear at the upper end of the resistor its when signal energy is transferred to the coupling coil l5l is applied to the suppressor grid Q8 through a filter resistor I55 provided with bypass condensers Hill and I59, connected between the ends of resistor I355 and ground, to perform the same functions as the corresponding filter condensers in the preceding circuits.

From the foregoing description and explana tion, it will be understood that the widest range of control of selectivity is obtained at the last intermediate frequency tuned circuit. Therefore, assuming that, with no signal impressed upon the antenna, the intermediate frequency coupling circuits 8?? and H23 have the same selectivity, it will be apparent that as the signal strength increases to make the volume and selectivity control efiective, the tuning of the second circuit H23 becomes broader than that of the preceding circuit 8i, although the tuning of the circuit Si is broadened appreciably.

While, in general, the tuning of the radio frequency circuits will be broader than the tuning of the intermediate frequency circuits, it may be desirable to so adjust the circuits that for very strong local signals the tuning of the last intermediate frequency circuit is broader than the tuning of the radio frequency circuits. This can easily be accomplished since the tuning of the intermediate frequency circuits will be broadened very rapidly with increase in signal strength in comparison with the radio frequency circuits Where the control voltage is necessarily small.

The audio signal output of the second detector 1 is transferred to an audio frequency amplifier (not shown) by means of an audio frequency transformer lti having a primary winding I53 connected between the anode l3! and the positive terminal of the voltage divider 3 i.

In the second detector l, the suppressor grid iiiii is connected directly to the cathode 529 in the conventional manner. 7

The screen grids. l3, d9, 9'5, and i339 of the vacuum. tubes 5, 3, and l, respectively, are supplied with the proper positive potential through conductors which are connected to a point on the voltage divider 3E. The abovementioned screen grids are provided with a radio ir-eqency connection to ground through a bypass condenser 565.

Although the diode rectifier circuits have been shown inductively coupled to the plate circuits in Fig. 1, any suitable type of coupling may be employed. Two satisfactory coupling circuits are shown in Figs. 2 and 3, where parts similar to those in Fig. 1 are indicated by like reference numerals.

Referring to Fig. 2, the diode rectifier of tube 5 is coupled to the output circuit of tube 3 by means of a coupling condenser ital connected between the anode 53 of tube 3 and the rectifier element H33 of tube 5. It will be seen that the remainder of the circuit is the same as the circuit between tubes 3 and 5 in Fig. 1 except that the coupling coil H3 and the filter and bypass condenser E9 have been omitted. The main difference between the rectifier circuit shown in Fig. 1 and that shown in Fig. 2 is that in Fig. 1 the rectifier is provided with a series feed connection, while in Fig. 2 it is provided with parallel feed connection, both of which are known and well understood.

Referring to Fig. 3, the rectifier in tube 5 is provided with a parallel feed connection by means of a coupling condenser I61 as in Fig. 2, the distinction between the two circuits being that in Fig. 2 the plate voltage is fed to anode 53 through the tuned coupling circuit Bl, while in Fig. 3 the voltage is fed to anode 53 through a coupling impedance I69 which may be a resistor, as shown.

In Fig. 3, the coupling condenser I05 is positioned between the anode 53 and the coupling circuit til whereby the negative biasing potential of resistor i539 may be applied to the grid 95 through the circuit 81. The coupling condenser iii? is connected between the upper terminal of circuit 8? and the rectifier element Hi3.

The type of coupling illustrated in Figs. 2 and 3 may be employed in all the amplifier and detector stages having automatic volume and selectivity control circuits, or the rectifiers may be parallel fed in certain stages and series fed in other stages.

Various other modifications may be made in my invention without departing from the spirit and scope thereof, and I desire, therefore, that only such limitations shall be placed thereon as ar necessitated by the prior art and are set forth in the appended claims.

I claim as my invention:

1. In a radio receiver comprising a plurality of cascade-coupled amplifiers, each amplifier including an electric discharge tube, the method of preventing overloading of the tubes in. the first amplifier stages Which comprises rectifying a received signal to change both the gain and the plate impedance less in the tubes which first amplify said signal than in the succeeding tubes, in response to an increase in the strength of a received signal.

2. A radio receiver comprising a plurality of cascade-coupled amplifiers, each amplifier including an electric discharge tube having 2. rectifier unit therein, means including said rectifier unit for so decreasing the gain and lowering the plate impedance of said tubes in response to an increase the strength of a received signal that the tubes which first amplify the signal have their gain and plate impedance decreased less than in the succeeding tubes.

3. In a radio receiver, a plurality of cascadecoupled amplifier tubes, each amplifier tube including an amplifier portion and a rectifier unit, means for supplying each of said rectifier units with signal energy whereby said energy is rectified, and means for utilizing said rectified energy for controlling the plate impedance of an amplifier portion of a preceding tube.

4. A radio receiver according to claim 3 characterized in that each rectifier unit is supplied with signal energy from a preceding tube.

5. In a radio receiver, a plurality of cascadecoupled amplifier tubes, each amplifier tube including an amplifier portion and a rectifier unit, means for supplying each of said rectifier units with signal energy from the amplifier portion of a preceding tube whereby said energy is rectified, and means for controlling the gain of the amplifier portion of said preceding tube by means of said rectified energy.

6. A radio receiver comprising a vacuum tube having a cathode, a control grid, a suppressor grid, and an anode, means for impressing a signal voltage across said cathode and said control grid,

an output circuit for said tube, said output circuit including said anode, a second vacuum tube having a cathode, a control grid, a suppressor grid,

an anode, and a rectifier electrode positioned adjacent to said cathode, .means for coupling said output circuit to the cathode and control grid of said second tube, a rectifier circuit connected in series with said rectifier electrode and adjacent cathode, said rectifier circuit including a resistor, means for supplying signal energy from said output circuit to said rectifier circuit whereby a voltage drop is produced in said resistor, and means for so connecting said resistor to said suppressor grid that when said voltage drop increases in value the suppressor grid is made more negative.

'7. A radio receiver including at least three cascade-coupled amplifiers having coupling circuits which are tuned, said receiver being characterized in that the tuning of each coupling circuit is broader than the tuning of a preceding coupling circuit, and in that means is provided for broadening the tuning of the coupling circuits in response to an increase in the strength of a received signal.

8. A radio receiver including at least three cascade-coupled amplifiers having coupling circuits which are tuned, said receiver being characterized in that the tuning of each coupling circuit is broader than the tuning of a preceding coupling circuit, and in that means is provided for both decreasing the gain of said amplifiers and broadening the tuning of the coupling circuits in response to an increase in the strength of the received signal.

9. A radio receiver including a plurality of cascade-coupled amplifiers having coupling circuits which are tuned, and means for controlling the gain of at least one of said amplifiers in response to a change in the strength of an incoming signal, said means including a circuit coupled to one of said amplifiers for taking energy therefrom, said receiver being characterized in that the tuning of each coupling circuit preceding said lastnamed amplifier is made broader than the tuning of a. preceding coupling circuit.

10. In a radio receiver comprising a plurality of cascade-coupled amplifiers having coupling circuits which are tuned, the method of preventing overloading of the first amplifiers which comprises broadening the tuning of said coupling circuits in response to an increase in the strength of an incoming signal and broadening the tuning of the later coupling circuits more rapidly than the tuning of the preceding coupling circuits.

11. In a radio receiver, a plurality of cascadecoupled amplifiers having coupling circuits which are tuned, means for decreasing the selectivity of one of said coupling. circuits a certain amount in response to a given increase in signal strength, and means for decreasing the selectivity of a succeeding coupling circuit by an amount greater than said certain amount in response to said increase in signal strength.

12. In a radio receiver, an amplifier including a plurality of tuned coupling circuits in cascade arrangement, means for so broadening the tuning of one of said coupling circuits in response to an increase in the strength of a received signal that said one coupling circuit is tuned broader than a preceding coupling circuit, said means comprising an automatic volume control circuit having an input circuit and an output circuit, said input circuit being coupled to said one coupling circuit, and said output circuit being coupled to a point in said amplifier preceding said one coupling circuit.

13. In a radio receiver, a plurality of cascadecoupled amplifier tubes, each of said tubes including a screen grid, an anode, and a grid positioned between said screen grid and said anode, a plurality of rectifiers, means for coupling one of said rectifiers to the output circuit of one of said amplifier tubes, means for coupling another of said rectifiers to the output circuit of another of said amplifier tubes, means including said one rectifier for controlling the potential of said lastnamed grid of one of said amplifier tubes, and. means including said other rectifier for controlling the potential of said last-named grid of another of said amplifier tubes.

KENNETH A. CHITTICK. 

